MIPS: kdump: Mark cpu back online before rebooting

The crash utility initializes cpu state by reading the system kernel
memory, which is copied into vmcore.

It is also natural to preserve the online state for CPUs at crash.

Failing to do so could make the analysis tool present info for only 1 CPU
by default, and unable to find panic task.

Signed-off-by: Dengcheng Zhu <dzhu@wavecomp.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20809/
Cc: Paul Burton <pburton@wavecomp.com>
Cc: "ralf@linux-mips.org" <ralf@linux-mips.org>
Cc: "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>
Cc: "rachel.mozes@intel.com" <rachel.mozes@intel.com>